Effect of Low-Concentration Polymers on Crystal Growth in Molecular

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Effect of Low-Concentration Polymers on Crystal Growth in Molecular Glasses: A Controlling Role for Polymer Segmental Mobility Relative to Host Dynamics Chengbin Huang, Charles Travis Powell, Ye Sun, Ting Cai, and Lian Yu J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11816 • Publication Date (Web): 31 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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The Journal of Physical Chemistry

Effect of Low-Concentration Polymers on Crystal Growth in Molecular Glasses: A Controlling Role for Polymer Segmental Mobility Relative to Host Dynamics Chengbin Huang 1, C. Travis Powell 2, Ye Sun 2, Ting Cai 3, Lian Yu 1,2,* 1 2

School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53705, United States. Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53705, United

States.

3

State Key Laboratory of Natural Medicines, Jiangsu Key Laboratory of Drug Discovery for

Metabolic Diseases, Department of Pharmaceutics, College of Pharmacy, China Pharmaceutical University, Nanjing 210009, China

Abstract. Low-concentration polymers can strongly influence crystal growth in small-molecule glasses, a phenomenon important for improving physical stability against crystallization. We measured the velocity of crystal growth in two molecular glasses, nifedipine (NIF) and o-terphenyl (OTP), each doped with 4 or 5 different polymers. For each polymer, the concentration was fixed at 1 wt % and a wide range of molecular weights was tested. We find that a polymer additive can strongly alter the rate of crystal growth, from a 10-fold reduction to a 10-fold increase. For a given polymer, increasing molecular weight slows down crystal growth and the effect saturates around DP = 100, where DP is the degree of polymerization. For all the systems studied, the polymer effect on crystal growth rate forms a master curve in the variable (Tg polymer – Tg host)/Tcryst, where Tg is the glass transition temperature and Tcryst is the crystallization temperature. These results support the view that a polymer’s effect on crystal growth is controlled by its segmental mobility relative to the host-molecule dynamics. In the proposed model, crystal growth rejects impurities and creates local polymer-rich regions, which must be traversed by host molecules to sustain crystal growth at rates determined by polymer segmental mobility. Our results do not support the view that host-polymer hydrogen bonding plays a controlling role in crystal growth inhibition.

1 ACS Paragon Plus Environment


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Introduction Glasses are amorphous solids formed by cooling liquids, drying solutions and condensing vapors while preventing crystallization. Glasses combine the mechanical strength of crystals and the spatial uniformity of liquids with numerous applications ranging from optics to structural materials. While better known glasses are inorganic, polymeric and metallic, there is increasing interest in organic glasses of relatively low molecular weight (“molecular glasses”) for applications in pharmaceutics, 1 biopreservation2 and electronics.3 In all their applications, amorphous materials must resist crystallization. While this requirement is easily met by some glasses, others – notably molecular glasses – readily crystallize,4,5,6,7,8,9,10 making the inhibition of crystallization a major task in developing these materials. Crystallization has two elemental steps – nucleation and growth, which have different kinetics. In the context of controlling crystallization in molecular glasses, an important phenomenon is the strong inhibitory effect of polymers on the process of crystal growth.11,12,13,14,15,16 For example, at only 1 wt %, polyvinylpyrrolidone can slow crystal growth in nifedipine by a factor of 10,14,15 suggesting the potential to use polymers as a general method to stabilize organic glasses. At present, it is still unclear how to select the best polymer for this purpose and more important, how a polymer interferes with the process of crystal growth. Some propose that host-polymer hydrogen bonding plays a controlling role,12,13,16 while others suggest that the polymer’s segmental mobility is important.15 A deeper understanding in this area is important for producing stable amorphous materials and controlling crystallization using low-concentration additives. In this study, we systematically investigated the effect of several polymers with different molecular weights on crystal growth in the glasses of nifedipine (NIF) and o-terphenyl (OTP). NIF and OTP are model molecular glass-formers, which show relatively fast crystal growth in the glassy state.4,11 The


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utilized polymers were polybutadiene (PB), polyisoprene (PI), poly(ethylene glycol/oxide) (PEG/PEO), polystyrene (PS), poly(4-tert-butylstyrene) (PtBS) and polyvinylpyrrolidone (PVP). At the molecular weight and concentration (1 wt %) used, they are all miscible with the host molecules and except for the highest molecular weight used, are present in the dilute state (the polymer concentration is below the contact concentration c*).17,18,19,20,21 For each polymer, the molecular weight covered a wide range to provide a critical assessment of the polymer attributes that control the effect on crystallization. We find that at 1 wt %, the polymer additive can strongly alter the rate of crystal growth in the glasses of NIF and OTP, from a 10-fold reduction to a 10-fold increase. For a given polymer, increasing molecular weight systematically slows down crystal growth and the effect saturates around DP = 100, where DP is the degree of polymerization. For all the systems studied, the polymer effect on crystal growth rate forms a master curve in the variable (Tg polymer – Tg host)/Tcryst, where Tg is the glass transition temperature and Tcryst is the crystallization temperature. These results support the view that the polymer’s segmental mobility relative to host-molecule dynamics controls its effect on crystal growth. Our results do not support the view that host-polymer hydrogen bonding controls crystal growth inhibition. We suggest a tentative model for the polymer effect on crystal growth on the basis of fracture, surface diffusion, and host-polymer segregation on the surface.

Experimental Section Nifedipine (NIF) and o-terphenyl (OTP) were obtained from Sigma-Aldrich (St. Louis, MO) and used as received. Polystyrene (PS), polybutadiene (PB), and polyisoprene (PI) were purchased from Scientific Polymer Products Inc. Polyisoprene (PI) was obtained from Polymer Source Inc. and Sigma-Aldrich. Poly(4-tert-butylstyrene) (PtBS) was purchased from Polymer Source Inc. Poly (ethylene glycol) (PEG) or poly (ethylene oxide) (PEO) was purchased from Sigma-Aldrich. The dimer of vinyl pyrrolidone



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(“VP

dimer”)

was

from

Abbott

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Laboratories.

Polyvinylpyrrolidone (PVP) was from ISP Technologies, BASF, and GAF Chemicals. Figure 1 shows the molecular structures of the materials in this study and Table 1 collects the relevant information on the polymers used. Figure 2 shows the Tg of polymers used relative to the Tg of the host molecules. For the systems studied, the presence of 1 wt % polymer generally had only a small effect on the host Tg (< 1 K). For the oligomers of PB and PI (M < 1000 g/mole), a slightly larger depression of the host Tg was observed, up to 2 K.

Figure 1. Molecular structures of the materials in this study. NIF: nifedipine; OTP: oterphenyl; PB: polybutadiene; PI: polyisoprene; PEO: poly (ethylene oxide); PVP: polyvinylpyrrolidone; PS: polystyrene; PtBS: poly (t-butyl styrene).

NIF glasses containing polymers were prepared by cryomilling (SPEX CertiPrep 6750 with liquid nitrogen as coolant) followed by melting and cooling. 1 g of 10 wt % polymer-doped NIF glasses were cryomilled first and then diluted to 1 g of 1 wt % polymer-doped NIF glasses at 10 Hz for five 2-min cycles, each followed by a 2-min cool down. To make a sample for measuring crystal growth, 2-3 mg of NIF glasses was melted at 453 K on a round coverslip (diameter = 15 mm) for 1 min and then covered by another coverslip to make a sandwich sample. The sample was cooled to room temperature by contact with an aluminum block. Because of its photosensitivity, all NIF samples were shielded from light. To measure the crystal growth rates, the NIF samples were nucleated at 333 K and then stored in a desiccator at 303 K or 313 K maintained with an oven stable within ± 1 K. Faster crystal growth at higher temperatures was observed with a Linkam THMS 600 hot stage (temperature stability ± 0.1 K) in flowing nitrogen.


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OTP glasses containing 1 wt % polymer additive were prepared by direct dissolution. The correct amounts of OTP and polymer additive were weighed, mixed and heated in a sealed vial at 353 K for at least 48 h with intermittent shaking to obtain a clear solution. A drop of the OTP solution was pipetted on a coverslip and covered by another coverslip. To initialize crystallization, a piece of OTP crystal was pushed into contact with the edge of the OTP liquid at room temperature. The sample was cooled to 243 K in a Linkam THMS 600 cold stage and crystal growth was observed under nitrogen purge. Crystal growth rate was measured through an Olympus BH2-UMA polarized light microscope equipped with a digital camera. Each reported Figure 2. The glass transition temperatures Tg of the polymers growth rate is the average of 9-12 measurements

used in this work shown as functions of molecular weight. The Tg of the host molecular glass is shown as horizontal line.

in three separate samples. Differential Scanning Calorimetry (DSC) measurements were performed in crimped aluminum pans with approximately 4-6 mg materials, using a TA Q2000 differential scanning calorimeter. The samples were cooled and heated at 10 K/min under 50 ml/min N2 purge to determine the Tg. Polymorphs of NIF crystals were identified with a Raman microscope (Thermo Scientific DXR Raman microscope; 780 nm laser). The polymorph of NIF crystal matched the known Raman pattern of



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β NIF, 22 with the following identifying peaks: C−C−O stretch (1215 cm−1), and C=C stretch (1651 cm−1). Only one OTP crystal structure is known (Cambridge Structural Database Ref. Code: TERPHO02) and was observed.

Table 1. Properties of the polymers used.

Mw (g/mol) or range Polybutadiene 520 (PB) 940 2640 5020 56 K 242 K 400 K Polyisoprene 1100 (PI) 2450 4600 9870 18 K 38 K 176 K Poly (ethylene oxide) 200 (190-210) (PEO) 400 (380-420) 600 (570-630) 2000 (1900-2100) 3350 (3000-3700) 8 K (7-9 K) 20 K (16-20 K) 100 K (Mv) 8000 K (Mv) Polystyrene 201 (PS) 580 1110 1780 3680 8400 17 K 97 K 979 K Poly (t-butyl styrene) 2350 (PtBS) 33 K 156 K 1500 K Polyvinylpyrrolidone 224 (“VP dimer”) (PVP) 2000-3000 (“K12”)

Mw/Mn 1.11 1.08 1.05 1.04 1.01 1.06 1.43 1.1 1.02 1.04 1.03 1.04 NA 1.15 NA NA NA NA NA NA NA NA NA 1.02 1.07 1.12 1.09 1.08 1.05 1.04 1.01 1.06 1.07 1.04 1.08 1.35 NA NA

Tg (K) 162 167 175 174 175 175 174 191 199 204 207 209 207 208 203 223 231 245 256 253 245 226 220 204 255 307 326 349 364 367 377 378 336 417 422 424 217 375


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8000 (“K15”) 44-54 K (“K30”) 1-2 M (“K90”)

NA NA NA

393 437 449

Note: Tg values of PB from Hintermeyer et al.23; Tg values of PEO from Faucher et al.24; Tg values of PVP from Powell et al.15 All other data from this work.



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Results Polymer-Host Miscibility. Before measuring its effect on crystal growth, we confirmed that each polymer used was miscible with the host molecules at the concentration used (1 wt %). This conclusion was based on the systematic change of the mixture’s glass transition temperature Tg and melting point Tm with concentration.15,25,26 Figure 3a illustrates the Tm test for NIF doped with PB 400K: Tm changes from pure NIF, to 1 wt % PB 400K doped NIF, to 10 wt % doped NIF, consistent with full miscibility at 1 wt % between the components. In addition to the Tg and Tm tests, the dependence of crystal growth rate on polymer concentration was also used to evaluate the state of mixing. For miscible systems,

the

crystal

growth

rate

u

decreases

exponentially with the polymer concentration (log u is linear on c). This is illustrated in Figure 3b for NIF

Figure 3. (a) Melting endotherms of NIF crystals in the presence of 1 and 10 wt % PB 400K at a 2 K/min heating rate. The results show that at 1 wt %, the polymer is miscible with NIF. (b) Crystal growth rates of NIF at 303 K in the presence of PS 17K, PS 97K and PVP K15 as functions of polymer concentration. The results indicate that PVP K15 is miscible with NIF, PS 17K is miscible up to ~ 1wt %, and PS 97K is immiscible at 1 wt %.

doped with PVP K15.14,15 In contrast, the corresponding data on NIF doped with PS exhibit plateaus. We interpret these results to mean that PS 17K is miscible with NIF up to ~ 1 wt %, whereas PS 97K is immiscible at 1 wt %. Because of this, PS 97K was not used in crystallization studies, but PS 17K was.


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Effect of polymer additives on crystal growth: NIF. Figure 4 shows typical data collected in this work. The process of crystal growth was followed in a pure NIF glass at 303 K and in NIF glasses containing 1 wt % PI 1100 and 176 K. These crystals are polycrystalline spherulites and their growth rate is constant in time (Figure 4d). Relative to pure NIF, the presence of 1 wt % PI (a low Tg polymer; see Figure 2) significantly speeds up NIF crystal growth. The rate increase depends on the PI molecular weight: by a factor of 20 for PI 1100 and a factor of 5 for and PI 176 K. Figure 5 summarizes the data collected on the effect of 1 wt % polymer on crystal growth in NIF glasses. Crystal growth rates u are shown for two temperatures of crystallization, Tcryst = 303 and 313 K, and plotted as functions of the polymers’ molecular weights. The u value for a pure NIF glass is shown as horizontal line. Notice a wide range of effects of a polymer additive on the crystal growth rate: PB, PI and PEO speed up crystal growth, whereas PVP and PS slow down crystal growth. It is noteworthy that at such a low polymer concentration Figure 4. Process of crystal growth in an NIF glass at 303 of 1 wt %, the crystal growth rate in an NIF can be

K (a) and an NIF glass containing PI 1100 (b) and PI 176 K (c). (d) Distance of crystal growth vs. time.



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increased or decreased by one order of magnitude. Notice also that the polymer’s molecular weight has a strong effect on the NIF crystal growth rate. For a given polymer, increasing the

molecular

weight

systematically

decreases the crystal growth rate and the effect saturates near Mw = 10 kg/mol. All the polymers tested show virtually the same molecular weight dependence, at both temperatures of crystallization (303 and 313 K).

Figure 5. Crystal growth rates in NIF glasses doped with 1 wt % polymers of different structures and molecular weights. (a) 303 K. (b) 313 K. For each temperature, the growth rate in pure NIF glass is shown as horizontal line.


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Effect of polymer additives on crystal growth: OTP. Figure 6 shows typical data collected for polymer-doped OTP glasses. The process of crystal growth was followed in a pure OTP glass at 243 K and in OTP glasses containing 1 wt % PI 1100 and 176 K. As in the case of NIF (Figure 4), the crystals formed are polycrystalline spherulites with constant growth rates (Figure 6d). Relative to pure OTP, the presence of 1 wt % PI 1110 has little effect on crystal growth rate, while 1 wt % PI 176K slows down OTP crystal growth. Figure 7 summarizers the data collected on the effect of 1 wt % polymer on crystal growth in OTP glasses. Crystal growth rates u are plotted as functions of the polymers’ molecular weights. The u value for a pure OTP glass is shown as horizontal line. Notice that a polymer additive generally slows down crystal growth in OTP, with the exception of PB whose oligomers speed up crystal growth. For PS Figure 6. Process of crystal growth at 243 K in a pure

and PtBS, the inhibitory effect is very strong on OTP glass (a) and an OTP glass doped with PI 1100 (b) and PI 176 K (c). (d) Distance of crystal growth vs. time.

crystal growth, reducing the growth rate by more than one order of magnitude.



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As in the case of NIF, the polymer’s effect on OTP crystal growth varies with its molecular weight. For a given polymer, increasing the molecular weight decreases the crystal growth rate and the effect saturates near Mw = 10 kg/mol for PB and PI and near 100 kg/mol for PS and PtBS. The overall pattern is similar to that observed for NIF (Figure 5).

The latter point is further demonstrated in

Figure 7. Crystal growth rates in OTP glasses doped with 1 wt % polymers of different structures and molecular weights. The growth rate in pure NIF glass is shown as horizontal line.

Figure 8, where the results on NIF and OTP are plotted together. To emphasize the molecular weight dependence, we normalize each crystal growth rate (u) by u∞, the growth rate plateau in Figures 5 and 7 reached at high polymer molecular

weight.

This

normalization

is

equivalent to vertically shifting the data points to coincide at high molecular weights. Figure 8 shows that after this normalization, all the data Figure 8. Normalized crystal growth rates in NIF and OTP

points cluster together, forming a common glasses vs. the polymer’s degree of polymerization. Note a trend. For this plot, we have used the degree of

similar dependence on DP for all polymers tested in both molecular glasses.

polymerization DP (DP = molecular weight of the polymer/molecular weight of the monomer) instead of molecular weight as the x axis because it slightly improves the collapsing of data. The general pattern


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revealed by Figure 8 is that for the systems studied, increasing the polymer’s DP decreases the rate of crystal growth rate and the effect saturates around DP = 100. Figure 9 shows another instructive way our results can be organized. Here we measure the polymer effect on crystal growth using the ratio u/uhost, where uhost is the crystal growth rate of the pure molecular glass and u is the value in the presence of a polymer. This ratio is plotted against (Tg host)/Tcryst,

and Tg host,

host

polymer–Tg

where host = NIF or OTP, Tg polymer are the Tgs of the polymer and the Figure 9. Polymer effect on crystal growth measured by u/uhost

respectively,

and

Tcryst

is

the

plotted against (Tg polymer–Tg host)/Tcryst, where Tcryst is the crystallization temperature. All the results of this study are organized into a master curve in this manner.

crystallization temperature. In this format, all the results of this study are organized into a master curve. The points above the horizontal line correspond to the cases where the polymer accelerates the crystal growth of host molecules, and those below to the cases where the polymer has an inhibitory effect. Figure 9 indicates that the polymer effect on crystal growth correlates strongly with the ratio (Tg polymer –Tg host)/Tcryst. It is significant that this master curve includes all the systems studied, including two hosts, three temperatures of crystallization, and 4 or 5 polymer additives in each host with wide ranges of molecular weights. As we discuss later, the ratio (Tg polymer–Tg host)/Tcryst measures the polymer’s segmental mobility relative to the host dynamics at the temperature of crystallization.



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We close the Results section with a comment on a peculiar behavior of PS-doped OTP glasses. At some PS fractions, two rates of crystal growth were observed. Figure 10a/b shows this phenomenon for the PS 8400 fraction: in the fast mode, the growth rate u is four times larger than in the slow mode. This phenomenon is observed in the Mw range 2 – 17 kg/mol, but not at lower or high Mw (Figure 10c). In the fast mode, u is insensitive to the PS molecular weight, whereas in the slow mode, it is. At present, the mechanisms for the two crystal growth modes are still unclear. The foregoing analysis (Figures 7-9) has focused on the slow mode in the PS-OTP system.

Figure 10. (a) Fast and slow modes of crystal growth in PS-doped OTP at 243 K. (b) Growth distance vs. time for the two growth modes. (c) Crystal growth rate in an OTP glass at 243 K containing 1 wt % PS of different molecular weights. Two rates are observed (fast and slow) for PS 1780, 3680, 8400 and 17K, and only one for other PS fractions.


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Discussion This study has examined the effect of low-concentration polymers on crystal growth in two molecular glasses (NIF and OTP). At 1 wt %, the polymer can strongly alter the rate of crystal growth, from a 10fold reduction to a 10-fold increase. For a given polymer, increasing molecular weight slows down crystal growth and the effect saturates around DP = 100 (Figure 8). For all the systems studied, the polymer effect on crystal growth forms a master curve in the variable (Tg polymer – Tg host)/Tcryst (Figure 9). As we discuss here, these findings support the notion that the polymer’s segmental mobility relative to the host-molecule mobility controls crystal growth in molecular glasses. We propose a tentative model for this effect. The fact that a polymer’s effect on crystal growth saturates with increasing molecular weight M argues that its center-of-mass diffusion constant D or intrinsic viscosity [η] does not have a rate-limiting role in the process. For our systems, the polymer is in a dilute solution (no entanglement), and both D and [η] are expected to change monotonically with M: D roughly as M

-0.5

(Ref. 27 and 28) and [η] roughly as

M 0.5 (Ref. 29 and 30). These trends are predicted by standard models.31 These trends, however, differ from the M dependence of a polymer’s effect on crystal growth (Figure 8), where a plateau is reached near DP = 100. It follows that D or [η] does not define the rate-limiting step for crystal growth in host molecular glasses. The same analysis above also suggests that the segmental mobility of polymer chains is a possible limiting factor for crystal growth. It is known that a polymer’s segmental mobility in a dilute solution decreases with increasing molecular weight but reaches a plateaus near DP = 100.32 This dependence is consistent with standard models.31 This molecular weight dependence of segmental mobility is in excellent agreement with that of a polymer’s effect on crystal growth (Figure 8).



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A further support for our conclusion is provided by the master curve in Figure 9, showing a strong correlation between a polymer’s effect on crystal growth and the ratio (Tg polymer - Tg host)/Tcryst. According to Angell,33 the relaxation time τ of a glass-forming molecular liquid is well described by log τ = log τg + m (Tg/T – 1)

(1)

where T is a temperature near Tg, τg is the relaxation time at Tg (~ 100 s), and m is a fragility index. For the two host liquids, m = 80 for OTP34 and 83 for NIF.35 These values are typical of glass-forming molecular liquids, though one should note a sizable variation in the literature values (a 20 % difference is not uncommon for a given system). This variation arises in part from the non-Arrhenius T dependence of τ, making its apparent activation energy sensitive to the choice of Tg. Eq. (1) also describes the segmental relaxation time τseg for a polymer melt with m having broadly similar values: m = 70-120 for PS,23 70-90 for PB,23 70-140 for PtBS,36 and 75-85 for PI.37 At the crystallization temperature Tcryst, the host-polymer mobility difference is given by log τseg/τhost = m (Tg polymer - Tg host)/Tcryst

(2)

Thus the ratio (Tg polymer – Tg host)/Tcryst is directly related to τseg/τhost, and the master curve in Figure 9 indicates a strong correlation between a polymer’s effect on crystal growth (measured by u/uhost) and the host-polymer mobility difference, where the polymer mobility refers to its segmental mobility. Note that the ratio τseg/τhost refers to the mobility difference between a neat polymer and a pure host at a common temperature. It might be more relevant to evaluate this property in the actual polymer solution. There is no direct experimental data at present on this property. The self-concentration model38 predicts that τseg of a polymer solute is not the same as the solvent dynamics τseg, but “remembers” its intrinsic mobility in the neat state. This is because for a given polymer segment, the local concentration of polymer segments exceeds the overall bulk concentration. This is a consequence of chain connectivity. The selfconcentration effect means that τseg of the neat polymer can be used to rank τseg in a dilute solution. 16 ACS Paragon Plus Environment

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How does the polymer’s segmental mobility control crystal growth in a molecular glass? Here we give a tentative answer with the aid of Figure 11. Our model is based on two recent studies. At the temperatures studied, crystal growth in NIF and OTP glasses is in so called “glass-to-crystal” or GC mode.4,5,11 In this process, crystal growth is not limited by the structural relaxation time in the amorphous phase. Among its various explanations,4,5, 39 , 40 , 41 ,42 the most recent by Powell et al. 42 hypothesizes that GC growth involves creation of voids and free surfaces (possibly through fracture) and the surface transport of molecules. Their proposal is based on the observation that volume is conserved during GC growth despite a higher density Figure 11. Model for crystal growth in

of the crystal. They reasoned that crystal growth must create molecular glasses containing dilute voids and free surfaces, which in turn accelerate local

polymers (black crystallization flux.

circles).

Fc

is

transformation. Their proposal is consistent with the fact that the velocities of crystal growth on the free surface and in the bulk have similar temperature dependence. A second result underlying our moldel is the finding26 that 1 wt % polystyrene in OTP can strongly inhibit the decay of surface gratings. A corrugated surface grating flattens under the driving force of surface tension, with molecules migrating from the peaks of the grating (regions of high chemical potential) to the valleys (low chemical potential). Zhang et al. suggest that in this process, slow-diffusing polymer molecules are stranded and enriched in regions vacated by host molecules. As shown in Figure 11, we hypothesize that free surfaces are constantly created during crystal growth, perhaps by fracture. The free surface allows fast transport of molecules to join the crystal. This process



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is known to create a depletion zone near a crystal,43 also depicted in Figure 11. We imagine that polymer molecules are rejected by the growing crystal, accumulating near the crystal/glass interface.26 Eventually, host molecules must migrate through a polymer-enriched region to reach the crystal. Based on previous work,44,45,46,47 translational diffusion of small molecules through a polymer matrix is limited by the segmental mobility of polymer chains. If the passage of host molecules through polymer-enriched regions becomes rate-limiting, we would expect a strong correlation between a polymer’s effect on crystal growth and its segmental mobility, as observed in this study.

Does host-polymer hydrogen bonding control crystal growth in molecular glasses? It has been proposed that host-polymer hydrogen bonding has a controlling effect on crystal growth in molecular glasses.13,16 Here we evaluate this proposal against the results of this work. Of the two host molecules, NIF is a donor and an acceptor of hydrogen bonds, while OTP is neither. Among the utilized polymers, PVP and PEO are acceptors; the rest (PB, PI, PS, and PtBS) are neither donors nor acceptors. The hydrogen-bonding model makes predictions only for NIF-polymer systems. The strength of hydrogen bonding with NIF is PVP (strongest) > PEO > PB ~ PI ~ PS (none). However, the inhibitory effect of a polymer on crystal growth follows the order: PVP > PS > PEO > PI > PB. The latter order is different from the order for host-polymer hydrogen bonding. In particular, PEO forms hydrogen bonds with NIF but accelerates its crystal growth; PS forms no hydrogen bonds with NIF but inhibits its crystal growth. A second test of the hydrogen-bonding model is to examine how molecular weight affects a polymer’s effect on crystal growth. Since it makes no reference to molecular weight, this model predicts the same effect independent of the degree of polymerization. Our results, however, show that the effect of a polymer on crystal growth significantly increases with molecular weight and reaches a plateau near DP = 100. The hydrogen-bonding model therefore does not correctly describe the systems studied. If a


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single variable is to be used to predict crystal growth inhibition, the ratio (Tg

polymer

– Tg

host)/Tcryst

performs much better (Figure 9) than host-polymer hydrogen bonding. The hydrogen-bonding model has the further shortcoming that it makes no predictions for systems without hydrogen bonds.

Conclusion In this study, we measured the crystal growth rate of amorphous NIF and OTP in the presence of 1 wt % polymer additives with different structures and molecular weights. We found that 1 wt % polymer additives can strongly alter the rate of crystal growth, from a 10-fold reduction to a 10-fold increase. For a given polymer, increasing molecular weight systematically slows down crystal growth and the effect saturates around 10-100 kg/mol. For all the systems studied, the polymer effect on crystal growth rate forms a master curve as a function of (Tg polymer–Tg host)/Tcryst. These results argue that center-of-mass diffusion or intrinsic viscosity of polymer chains does not define the rate-limiting step for the crystallization of host molecules, while segmental mobility likely plays this role. This view is consistent with all the host-polymer systems studied and serves as a bases for prediction. On the basis of crystallization-induced fracture and surface diffusion, a tentative model is proposed to explain why a polymer’s segmental mobility controls the crystal growth of host molecules.

ACKNOWLEDGEMENT. We thank the NSF (DMR-1234320) for supporting this work and M. D. Ediger for helpful discussions. C.T.P. thanks the NSF for a graduate research fellowship.

Corresponding Author. Lian Yu. Telephone: 608-263-2263. E-mail: [email protected].



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TOC Graphic


Effect of Low-Concentration Polymers on Crystal Growth in Molecular

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